Fix processing of very short messages in DKIM.
Fix processing of very short messages in DKIM.

Reported by:	@citrin

amohanta/rspamd,awhitesong/rspamd,andrejzverev/rspamd,minaevmike/rspamd,andrejzverev/rspamd,andrejzverev/rspamd,AlexeySa/rspamd,AlexeySa/rspamd,minaevmike/rspamd,andrejzverev/rspamd,minaevmike/rspamd,AlexeySa/rspamd,amohanta/rspamd,AlexeySa/rspamd,andrejzverev/rspamd,dark-al/rspamd,AlexeySa/rspamd,amohanta/rspamd,minaevmike/rspamd,awhitesong/rspamd,amohanta/rspamd,dark-al/rspamd,minaevmike/rspamd,minaevmike/rspamd,AlexeySa/rspamd,dark-al/rspamd,minaevmike/rspamd,AlexeySa/rspamd,andrejzverev/rspamd,amohanta/rspamd,dark-al/rspamd,andrejzverev/rspamd,awhitesong/rspamd,minaevmike/rspamd,minaevmike/rspamd,AlexeySa/rspamd,dark-al/rspamd,andrejzverev/rspamd,AlexeySa/rspamd,awhitesong/rspamd